from collections import Counter
d1={'a':2,'b':5,'c':7}
d2={'c':3,'b':3,'d':4}
d3={'c':3,'b':3,'d':4}
c_d1,c_d2=Counter(d1),Counter(d2)
c_d3=Counter(d3)
dict_merge=dict(c_d1+c_d2+c_d3)
print(dict_merge)